The Iranian islamic revolution refers to events involving the overthrow of the Pahlavi dynasty under Mohammad Reza Shah Pahlavi, who was supported by the United States  and its eventual replacement with an Islamic republic under the Grand Ayatollah Ruhollah Khomeini, the leader of the revolution, supported by various  Islamic organizations and Iranian student movements.Demonstrations against the Shah commenced in October 1977, developing into a campaign of civil resistance that was religious based  and intensified in January 1978. Between August and December 1978 strikes and demonstrations paralyzed the country. The Shah left Iran for exile on January 16, 1979, as the last Persian monarch, leaving his duties to a regency council and an opposition-based prime minister. Ayatollah Khomeini was invited back to Iran by the government, and returned to Tehran to a greeting by several million Iranians. The royal reign collapsed shortly after on February 11 when guerrillas and rebel troops overwhelmed troops loyal to the Shah in armed street fighting, bringing Khomeini to official power. Iran voted by national referendum to become an Islamic Republic on April 1, 1979, and to approve a new theocratic-republican constitution whereby Khomeini became Supreme Leader of the country, in December 1979.The revolution was unusual for the surprise it created throughout the world: it lacked many of the customary causes of revolution (defeat at war, a financial crisis, peasant rebellion, or disgruntled military), occurred in a nation that was enjoying relatively good material wealth and prosperity, produced profound change at great speed, was massively popular, resulted in the exile of many Iranians, and replaced a pro-Western semi-absolute monarchy  with an anti-Western authoritarian theocracy based on the concept of Guardianship of the Islamic Jurists.